<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>什么是rem</title>
</head>
<body>
    <div class="parent">
        <div class="son"></div>

    </div>
</body>
<style>

    html{
        font-size: 10px;
    }

    body{
        font-size: 20px;
    }

    .parent{
        width: 20rem;
        height: 20rem;
        font-size: 20px;
    }
    .son{
        width: 15rem;
        height: 15rem;
        background-color: aqua;
    }
</style>
</html>